A fuel assembly in a boiling water nuclear reactor includes an elongated tubular container, often with a rectangular or square cross section, which is open at both ends forming a continuous flow passage, through which the coolant of the reactor is able to flow. The fuel assembly comprises a large number of also elongated tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. At the top, the fuel rods are retained by a top tie plate and at the bottom by a bottom tie plate. To allow coolant in the desired manner to flow past the fuel rods, it is important that these be kept at a distance from each other and prevented from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction.
When coolant flows upwards through the core, the flow induces force components which influence the fuel rods in a horizontal direction such that these start vibrating. These vibrations may give rise to abrasion damage on the rods. The abrasion damage occurs primarily at those points where the rods make contact with the internal fixed or resilient support of the spacer cell. In difficult cases, the abrasion may cause penetration of the fuel rod, allowing fissile material to pass out into the core.
It is known to increase the margin with respect to abrasion by reducing the distance between the spacers. This, however, causes another problem, namely, that the spacers must not have such a high individual flow resistance that the total pressure drop exceeds a predetermined value during the passage of the coolant through the fuel assembly.
The design of spacers means that contradictory requirements must be taken into consideration. On the one hand, the spacer shall be sufficiently strong to reduce the deflection and vibration of the fuel rods and to resist great thermal and hydraulic forces. It must provide a sufficient contact surface with the fuel rods to minimize local wear on the fuel rods at the contact points. It must be able to withstand swelling of the fuel rods. On the other hand, the spacer shall be designed with a minimum amount of material to minimize the neutron-absorbing effect. It shall be designed so as to provide a minimum flow resistance and the material from which it is manufactured shall be low-absorbing, for example zirconium.
Another important factor when designing a spacer for a boiling water reactor is to ensure that the spacer effects a good cooling of the fuel rods by a suitable mixing of the coolant. In those cases where the cooling is not sufficient, so-called dryout may arise. In serious cases, dryout gives rise to penetration of the fuel rods.
The smallest permissible dryout margin during stationary reactor operation shall have a value which involves an acceptably small risk of fuel damage caused by dryout both during normal operation and expected transients.
The spacers thus influence the flow of the coolant and hence the cooling of the fuel. It is known that in a region immediately below the spacer, where the coolant has not yet passed the spacer, a deterioration takes place of the coolant film on the fuel rods, whereas in a region above the spacer, where coolant has just passed the spacer, a reinforcement of the water film instead takes place. The reinforcement of the coolant film is due to the turbulence which arises in the coolant when it passes a spacer. The greatest risk of dryout arises in the upper part of the fuel immediately below the spacers.
Known spacers often comprise a lattice of plate bands arranged crosswise and standing on end, these plate bands forming substantially square cells. Inside the cells there are usually arranged fixed and/or resilient supports for all-sided positioning of the fuel rods or the control rod guide tubes extending through the cells. It is known to design spacers for boiling water reactors with a lattice comprised of sleeves, where the sleeves, for example, are made of one or more materials with different spring rates. A lattice of two materials is shown in Swedish patent specification 469 047. A lattice comprised of sleeves provides more favourable cooling conditions than a lattice comprised of plate bands in that the sleeves are given an at least substantially circular shape which can better conform to the normally circular fuel rods. The better cooling properties are due to the fact that water tends to accumulate where the plate bands cross each other, that is, between the rods and at a distance therefrom, whereas in case of sleeve cells the water accumulates at the walls of the sleeve which are arranged nearer the fuel rods whereby the water can be utilized in a better way.
Another disadvantage of spacers with lattices of plate bands is that they are expensive and complicated to manufacture.
In CH 460 965, FIG. 8, a sleeve spacer is shown which has elongated embossments arranged at the corners of the sleeve. The shape of the sleeve with six wall parts means that when the sleeve is arranged in a hexagonal lattice, the wall parts will make close contact with each other except where the walls are provided with the embossments. This in turn means that the resilient property of the sleeve is limited to the resilience in the very embossment. The disadvantage of this embodiment is that only the resilience in the very embossment is to accumulate the forces which arise in rods and spacers.
Known spacers are often made completely or partially of Inconel which is a material with good strength and good properties with regard to machining when manufacturing spacers. However, it is desirable to manufacture spacers of a material with lower ability to absorb neutrons, that is, a low-absorbing material, for example a zirconium alloy. Spacers of zirconium alloy are an advantage from the point of view of reactivity economy. In addition, spacers of a low-absorbing material are less radioactive after irradiation, which is an advantage when handling the spacers after reactor operation.
One difficulty with zirconium alloys is that the material relaxes upon irradiation, which involves a considerable reduction of the spring constant in a short time. The fixed supports according to the prior art are usually designed with point contact with the fuel rod or with line contact which extends over a minor part of the height of the spacer. Point contact is unfavourable from the point of view of abrasion.
The fixed supports are usually supplemented by one or more resilient supports for all-sided positioning of the fuel rods extending through the cells.
The object of the invention is to provide a spacer which has a good margin with respect to abrasion, which achieves good cooling, and which is simple to manufacture and, in addition, is possible to manufacture of zirconium alloy.